Elements of a simple program

• Documentation -Comment using //
• // - for rest of line
• /* start of any length comment and end */
• Your name
• Class Period
• Due date of assignment
• Program description
• Variable dictionary of all variables declared and used by this module

Variable declaration

• var followed by name of variable (identifier)
• Identifiers
• Consist of letters, digits, & and _
• Cannot begin with digit
• Case sensitive (a1 and A1 are different)
• Cannot be a keyword or reserved word
• Use meaningful names
• All caps indicates a constant
• Declarations
  • var house_length ;
    var some_num, first_name;
    
  • Placement of variable declaration - almost anywhere
  • For now, place them immediately after start of javascript code
  • Results in storage allocation from RAM

Modifying the value of a variable:

1. Input statement
• window.prompt ("Message", "initial");
• wait for the user to enter a value
• place the value into the memory location associated with the value
2. Assignment statement
• General format: Variable = Expression ;
• perimeter = length + length + width + width;
• Semantics
• evaluate the right hand side (rhs)
• use operator precedence
• place result in memory for variable
• Potential error: Division by 0

More Assignment Statements

• Arithmetic Operator Precedence
  • Parentheses ()
  • Multiplication *, Division /, Modulus %
  • Addition +, Subtraction -/LI>
  • Mathematical Compatibility
    In order to "do math", need to convert string to numeric
    • use parseInt to convert to integer
      some_int = parseInt(some_int) ;
    • use parseFloat to convert to a decimal number
      some_float = parseFloat(some_float) ;

    CORRECT var counter, sum; counter = 20; counter = parseInt(counter); sum = counter + 6;

    INCORRECT var counter, sum; counter = 12; sum = counter + 6;

  • Literal is a name for one specific value. For example the number two is the literal 2

Output

• document.write (list of items separated by + );
• new lines in output "\n"
• document.write writes one line to the page
• document.writeln (list of items separated by + );
• document.write writes one line to the page followed by a br tag so that when the source code is viewd, it is neat